Diffraction type sound absorber with fibrous walls



v. T. FARE New 22 1949 DIFFRACTION TYPE SOUND ABSORBER WITH FIBROUS WALLS Filed Jan. 27, 1944 a INVENTOE. lz'zlw'l mfl HTTU/El/f) Patented Nov. 22, 1949 DIFFRACTION TYPE SOUND ABSORBER WITH FIBROUS WALLS Victor T. Par, Westmont, N. J assignor to Radio Corporation of America, a corporation of Delaware Application January 27, 1944, Serial No. 519,879

4 Claims. 1

This invention relates to a wall structure for acoustic absorbers, and more particularly to an acoustic absorber of the diffraction type, the present invention bein in the nature of an improvement over similar wall structures disclosed and claimed in the copending application of Harry E. Olson, Serial No. 512,320, filed November 30, 1943, and assigned to Radio Corporation of America.

In the aforesaid Olson application, there is disclosed an acoustic absorber comprising a casing which encloses a relatively large fluid space and the wall structure of which is constituted at least in part by a material which is pervious to acoustical waves. This material is of a character such that it offers a relatively large dissipative impedence to the passage of sound waves therethrough from the exterior of the casing to the interior, the space enclosed by the casing being of sufficiently large volume to offer a relatively small impedence to the passage of sound waves through the casing. In general, the wall structures proposed by Olson are of the laminated type, being constituted by a relatively rigid,

self-sustaining, outer, perforated layer which is itself impervious to sound waves but through the openings of which the sound waves may pass, and an inner layer of relatively limp material having a plurality of fine passages through which the sound waves may pass to the interior of the casing but in which the energy of the sound waves is dissipated. As disclosed by Olson, the two aforesaid layers are secured to each other by means of staples, adhesive, or the like, thus requiring an assembly step which adds to the cost of the absorber.

The primary object of my present invention is to provide an improved wall structure for acuostic absorbers of the type disclosed by Olson, which structure will be free from the aforementioned limitation.

More particularly, it is an object of my present invention to provide an improved, fibrous sound absorbing material which is self-supporting and which fulfills the requirement of a relatively dense, sound impervious outer lay, and a rela-- tively soft and fiuffy, sound impervious inner layer.

It is also an object of my present invention to provide an improved wall structure which is relatively inexpensive in cost and highly eflicient in use.

In accordance with the present invention, the sound absorbing material includes a first layer of fibrous material which is relatively rigid and self-supporting and which is impervious to the passage of sound waves therethrough except through openings provided therein. Over the first layer of fibers is a second layer of loosely arranged fibers which provide a relatively limp, soft, fluffy layer integral with the first mentioned layer and having a plurality of intertwined passages through which the sound waves can pass but which offer a large dissipative impedence to the sound waves. Thus, an integral wall structure suitable for the manufacture of absorbers of the type disclosed in the above identified Olson application may be produced easily and at low cost, and such wall structures will be found to be highly efficient in use.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing in which Figure 1 is a perspective view, partly in section, of a box-like acoustic absorber similar to that disclosed in the aforesaid Olson application, but formed of a wall structure of my present invention, and

Figure 2 is a View, in elevation, of a somewhat different form of acoustic absorber employing my improved wall structure.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig. 1, a box-like absorber l constituted by a wall structure 3 and enclosing a relatively large space 5 in accordance with the teachings of Olson. The wall structure 3, however, is formed in accordance with my present invention and is constituted by an outer layer 3a of densely or compactly arranged fibers backed by an inner layer 3b of relatively loosely arranged fibers. The wall structure 3 is provided with a plurality of substantially uniformly distributed openings 1 which extend at least through the outer layer 3a, although they may also extend somewhat into the inner layer 3b is desired.

The fibers of the layers 3a and 31) may be constituted by cotton linter, shredded wood, sulphite, kraft, or mixtures thereof, or the outer layer 311 may be formed of such fibers and the inner layer 31) may be formed of wool fibers with or without a mixture of shredded wood and the like, as may be found most desirable. In any case, the fibers in the outer layer 3a are arranged in sufficiently compact, mutual relationship to render the outer layer 3a rigid and self-sustaining. Thus, the outer layer 311. is impervious to the passage of sound waves therethrough, except through the openings 1, through which the sound waves may pass substantially freely. The fibers of the inner layer 31), on the other hand, are much more loosely arranged, as can be seen clearly from Fig. 1, and this layer is relatively soft, fluffy, and somewhat limp. By arranging the fibers of the layer 322 loosely, there are provided a plurality of intertwined, fine or thin passages through which sound waves reaching the layer 3b through the openings 1 may pass into the interior of the casing I. These fine passages are, however, of sufficiently small dimension to offer a large dissipative impedance to the passage of sound waves therethrough, and the energy of the sound waves incident upon the absorber I is therefore dissipated in the manner taught by Olson in his above-identified application.

In Fig. 2, there is shown a somewhat similar acoustic absorber l but formed by two substantially conical sections 9a and 9b. As in the case of the absorber shown in Fig. 1, the absorber of Fig. 2 has a wall structure constituted by an outer perforated layer of compactly or densely accreted fibers and an integral backing layer of relatively loosely accreted fibers, the conical casing sections 9a and 9b enclosing a relatively large space or cavity between them.

From the foregoing description, it will be apparent to those skilled in the art that I have provided an improved acoustic wall structure particularly suitable for acoustic absorbers of the type referred to above. Obviously, the layers 3a and 31) may be made as dense or as fiufiy as may be found necessary or desirable, and each layer can be built up to a desired thickness independently of the other.

Although I have shown and described but one embodiment of my invention, it will undoubtedly be apparent to those skilled in the art that many other variations thereof are possible. For example, instead of making the dense layer 3a the outer one and the fiuffy layer 31) the inner one, these layers may be reversed since the function of the rigid layer 3a is, essentially, to provide a support for the acoustical energy dissipating layer 31). Other similar variations will undoubtedly be apparent to those skilled in the art. I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A diffraction type acoustic absorber comprising a casing enclosing a fiuid filled space, said casing having a wall structure comprising an outer layer and an integral inner layer, one of said layers being formed'of relatively compactly arranged fibers and the other of said layers being formed of relatively loosely arranged fibers, the fibers of said one layer being arranged with such compactness that said one layer is substantially impervious to the passage of acoustical waves therethrough, said one layer having, however, a plurality of openings therein through which said waves can pass substantially freely, and the fibers of said other layer being arranged with sufficient looseness to provide a series of passages therebetween through which acoustical waves can pass but which oifer relatively large dissipative impedance to the passage of said waves therethrough.

2. A diffraction type acoustic absorber comprising a casing enclosing a fluid filled space, said casing having a wall structure comprising an outer layer and an integral inner layer, said outer layer being formed of relatively compactly arranged fibersand said inner layer being formed of relatively loosely arranged fibers, the fibers of said outer layer being arranged with such compactness that said outer layer is substantially impervious to the passage of acoustical waves therethrough, said outer layer having, however, a plurality of openings therein through which said waves can pass substantially freely, and the fibers of said inner layer being arranged with sufiicient looseness to provide a series of passages therebetween through which acoustical waves can pass but which offer relatively large dissipative impedance to the passage of said waves therethrough.

3. A diffraction type absorber according to claim 1 characterized in that said wall structure is constituted solely by said fibrous layers.

4. A wall structure for acoustic absorbers, said structure comprising a layer of compactly arranged fibers and an integral layer of relatively loosely arranged fibers, the fibers of said first named layer being sufficiently compact to render said first named layer relatively rigid and self-sustaining and the fibers of said second named layer being arranged to render said second named layer relatively limp, said structure having a plurality of openings therein of a size sufiicient to substantially freely pass acoustical energy therethrough, and said openings being distributed substantially uniformly over and extending at least through said first named layer but not through said second named layer.

VICTOR T. PARE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,190,133 Edison July 4, 1916 1,705,778 Munroe et al. Mar. 19, 1929 1,751,249 Rosenblatt Mar. 18, 1930 1,876,117 Weinberger Sept. 6, 1932 2,029,441 Parkinson 1 Feb. 4, 1936 2,081,952 Parkinson 1 June 1, 1937 2,100,395 Heymann Nov. 30, 1937 2,124,086 Slidell July 19, 1933 2,160,638 Bedell et al. May 30, 1939 2,362,859 Rosenblatt Nov. 14, 1944 FOREIGN PATENTS Number Country Date 409,502 Great Britain May 3, 1934 Certificate of Correction Patent N0. 2,488,555 November 22, 1949 VICTOR T. PARE It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 1, line 47, for impervious read peroious; column 2, line 46, for the Words 15 desired read cf desired;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 18th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent N0. 2,488,555 November 22, 1949 VICTOR T. PARE It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correctlon as follows:

Column 1, line 47, for impervious read perm'ous; column 2, line 46, for the words is desired read 'Ij desired;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the ease in the Patent Ofliee. V

Signed and sealed this 18th day of April, A. D. 1950 THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

